W.K. Teo

Preparation and characterization of polyvinylidene fluoride (PVDF) hollow fiber membranes

Abstract Polyvinylidene fluoride (PVDF) hollow fiber membranes were prepared by dry/wet and wet phase inversion methods. In spinning these PVDF hollow fibers, dimethylacetamide (DMAc) and polyvinyl pyrrolidone (PVP) were used as a solvent and an additive, respectively. Water was used as the external coagulant. Water or ethanol was used as the internal coagulants. The membranes were characterized in terms of water flux, molecular weight cut-off for the wet membranes. Gas permeation fluxes and effective surface porosity were determined by a gas permeation method for the dried membranes. The cross-sectional structures were examined by scanning electron microscopy. The effects of polymer concentration, air-gap, PVP molecular weight, PVP content in the polymer dope, and the internal coagulant on the permeation properties and membrane structures were examined. Highly permeable PVDF hollow fiber membranes could be prepared from a polymer dope containing low molecular weight PVP and using ethanol as the internal coagulant.

Porous PVDF asymmetric hollow fiber membranes prepared with the use of small molecular additives

Abstract Preparation of polyvinylidene fluoride (PVDF) asymmetric hollow fiber membranes was studied by introducing small molecular additives, which include nonsolvents (water, ethanol and 1-propanol) and inorganic salt (LiCl). Dimethylacetamide (DMAC) was used as a solvent. Water was used as an external coagulant, while water, ethanol or a mixture of water and ethanol was used as an internal coagulant. The prepared PVDF hollow fiber membranes were characterized in terms of water flux, and molecular weight cut-off for the wet membranes. Average pore size and effective surface porosity were determined using the gas permeation method for the dried membranes. The cross-sectional structure of the hollow fibers was examined by scanning electron microscopy. The collapsing pressure and wetting pressure of the dried membranes were also tested. The effect of polymer concentration, non-solvents, the mixture of non-solvent and LiCl, internal coagulant and post-treatment was studied in details. The PVDF hollow fibers prepared from the small molecular additives exhibit good mechanical strength and excellent hydophobicity. The PVDF hollow fiber spun from the non-solvent alone exhibits a quite low permeability, while good PVDF porous hollow fiber membranes were prepared using a mixture of the water/LiCl, or 1-propanol/LiCl as the additive.

Corrosion Mechanisms and Products of Copper in Aqueous Solutions at Various pH Values

Abstract The corrosion behavior of copper in aqueous solutions of different pH values was investigated using electrochemical and surface analysis methods. It was shown that the corrosion mechanism changed with pH and was associated with morphology of the surf ace films formed. In solution of pH 3, the copper surface was covered with porous corrosion products of cuprous oxide (Cu2O). Corrosion was controlled predominately by diffusion in solution. In solutions of pH 4 to pH 5, formation of cubic Cu2O on the copper surface provided a diffusion barrier to copper dissolution. The anodic process was controlled by a mixed diffusion of copper ions in oxide films and in solution. In solutions of pH 6 to pH 9, the oxide films (Cu2O) became more protective. Diffusion in the oxide films became a rate-determining step of anodic dissolution. In pH 10 solution, a thin, compact Cu2O film formed, and spontaneous passivation was observed. At pH 12 and pH 13, analysis by x-ray photoelectron spectroscopy (XPS) and scanning ...

Corrosion Protection of Copper by a Self‐Assembled Monolayer of Alkanethiol

A self-assembled monolayer of 1-dodecanethiol (DT) was formed on a copper surface pretreated using different methods. The corrosion protection abilities of the monolayer were evaluated in an air-saturated 0.51 M NaCl solution using various techniques including electrochemical impedance spectroscopy, polarization, coulometry, weight loss, and X-ray photoelectron spectroscopy. It was found that the corrosion resistance of the monolayer was improved markedly by using a nitric acid etching method. A minimum concentration of 10{sup {minus}4} M DT was needed to form a protective monolayer. The DT-monolayer retarded the reduction of dissolved oxygen and inhibited the growth of copper oxide in the NaCl solution. In comparison with other inhibitors, such as benzotriazole (BTA) and mercapto-benzothiazole (MBT), the DT-monolayer showed much better corrosion resistance in aqueous solution.

Polyethersulfone hollow fiber gas separation membranes prepared from NMP/alcohol solvent systems

Polyethersulfone (PESf) asymmetric hollow fiber membranes were prepared by the dry/wet and the wet-phase inversion processes from spinning solutions containing N-methyl-2-pyrrolidone (NMP) and an alcohol as a nonsolvent-additive (NSA). Two grades of polyethersulfones. Victrex 4800P and Radel A-300, were used. The alcohols used in this study include five aliphatic alcohols (methanol, ethanol, 1-propanol, 1-butanol, 1-pentanol) and two polyhydric alcohols (ethylene glycol and diethylene glycol). Water was used as both the internal and external coagulant in preparing these hollow fiber membranes. All spinning solutions were formulated such that their compositions were close to the point of incipient phase separation. The effects of the alcohols as NSA on the spinning solution properties; permeation fluxes of pure He, CO2, O2 and N2; and structures of the PESf hollow fiber membranes were investigated. The results show that different alcohols have a dramatically different influence on the polymer solution properties, gas separation characteristics and structures of the resulting membranes. Addition of a suitable alcohol (C2–C4 aliphatic alcohols) in the spinning solution greatly improved both permeability and ideal selectivity of the gases examined. The PESf hollow fiber membranes prepared from NMP / EtOH solvent systems exhibited the best gas separation characteristics comparable, if not better than those reported for other types of hollow fibers. Higher molecular weight alcohols, C5 and above aliphatic alcohols as well as polyhydric alcohols were found to be not suitable in making PESf membranes for gas separation using water as coagulant.

The synergistic effects of propargyl alcohol and potassium iodide on the inhibition of mild steel in 0.5 M sulfuric acid solution

Abstract The corrosion and inhibition behaviors of mild steel in aerated sulphuric acid in the presence of propargyl alcohol (PA) and potassium iodide (KI) were investigated using electrochemical methods. It was found that the inhibition efficiency increased with PA concentration, but the potential of unpolarizability (Eu), which indicates the commencement of desorption of adsorbed propargyl alcohol on the electrode, remained unchanged with increasing PA concentration. The addition of potassium iodide in the solution increased the value of Eu. A synergistic effect was observed between KI and PA with an optimum mass ratio of [PA]/ [KI]=1/1. The experimental results suggest that the presence of iodide ions in the solutions stabilized the adsorption of PA molecules on the metal surfaces and, therefore, improved the inhibition efficiency of PA. In the solutions containing propargyl alcohol (PA), the Eu values remained unchanged with increasing immersion times, indicating that there was less possibility of polymerization of PA molecules. An adsorption mechanism was favored for the inhibition of PA.

The corrosion behaviour of copper in neutral tap water. Part I: Corrosion mechanisms

Abstract The corrosion behaviour of copper in neutral aerated simulated tap water is investigated using electrochemical methods and XPS. Semi-infinite diffusion behaviour is found in electrochemical impedance spectroscopy (EIS) from intermediate to low frequency. Polarization resistance ( R p ) obtained at very low frequency (0.0005 Hz) increases with immersion time, which coincides with the growth of oxide film on copper. The growth of oxide film with immersion time has little effect on the cathodic process, but considerably reduces the anodic dissolution current. The observed results suggest that the diffusion of copper ions in oxide film controls the overall corrosion rate. Under rotating conditions polarization resistance decreases and anodic current increases, which was caused by the decrease of the oxide film thickness. The phenomena of the mixed diffusion of copper ion in oxide film and in solution has been observed at a low pH of 5. Passivation is found when pH = 10. XPS spectra show that the oxide film formed is composed mainly of cuprous oxide. An equivalent circuit representing the corroding interface is proposed with a discussion of theoretical approaches to the calculation of diffusion impedance and polarization resistance.

Selective removal of trace H2S from gas streams containing CO2 using hollow fibre membrane modules/contractors

Abstract Removal of trace H2S from gas streams containing high concentration of CO2 was carried out in a polyvinylidene fluoride porous asymmetric hollow fibre membrane module, using an aqueous solution containing 2 M sodium carbonate as absorption medium. Effects of gas/liquid flow ratio, CO2 concentration and operating pressures on the extent of H2S removal, H2S selectivity and overall mass transfer coefficients were investigated. The selectivity for H2S was about one order of magnitude higher than that in conventional-packed towers. Using feed gas mixtures containing 1000 ppm H2S with CO2 concentrations ranging from 5–23 vol.%, the H2S removal efficiency of nearly 100% was attained with less than 5 vol.% of CO2 permeated and absorbed. Experimental results indicate that the membrane module used was very efficient in the selective removal of trace H2S to ultra-low concentration even at high gas/liquid flow ratio.

Preparation and characterization of polyetherimide asymmetric hollow fiber membranes for gas separation

Integral asymmetric polyetherimide (PEI) hollow fiber membranes were prepared using N-methyl-2-pyrrolidone (NMP) as a solvent and ethanol as a nonsolvent-additive (NSA). Two polymer dopes with polymer concentrations 30 and 25 wt% and the coagulation value of 1.0 g were formulated. The mass ratio of ethanol to NMP in these polymer dopes was 0.2. Water was used as both an internal and external coagulant. Hollow fibers with an outer diameter of about 600 μm and an inner diameter of about 300 μm were prepared at different air-gaps. The membrane structure and gas separation properties were examined. The results reveal that the length of air-gap plays an important role in the spinning process and greatly affects the membrane separation performance. Non-coated asymmetric membranes with HeN2 selectivity as high as about 90 were obtained within an optimum range of air-gap lengths. After silicone-coating, the membranes prepared exhibit outstanding selectivity and moderate permeation flux. Gas permselection properties for N2, CH4, Ar, CO2, O2, H2, and He through the PEI membranes were investigated in some detail as a function of pressure and temperature.

Use of permeation and absorption methods for CO2 removal in hollow fibre membrane modules

Abstract Hollow fibre membrane modules have been investigated experimentally for removing CO2 from a gas stream using both permeation and absorption methods. These membrane modules consist of a bundle of hollow fibres made from either silicone rubber or polyethersulphone. The silicone rubber membrane is the homogenous type, while the polyethersulphone membrane is asymmetric type with a dense skin layer at the outer edge of the fibre. The gas mixture containing 4% CO2 was introduced into the hollow fibre lumen and in cocurrent or countercurrent flow with fluids in the module shell. Both experimental and simulation results have been obtained and the beneficial effect of the permeate gas absorption have been demonstrated. It has been shown that the use of water as an absorbing liquid in the permeate side of the modules can greatly improve the extent of CO2 removal. This improved carbon dioxide selectivity is attributed to the presence of the liquid film which, on the other hand, creates the liquid film resistance, resulting in much reduced CO2 permeation flux and hence the loading capacities of the modules. The loading capacities of the modules can be improved using an alkaline solution instead of neutral water.